1. Field of the Invention
The present invention relates to a servo control system having a function for improving a processing accuracy in a machine tool wherein a plurality of axes are cooperatively actuated.
2. Description of the Related Art
When a plurality of axes are cooperatively actuated in a machine tool, such as a machining center, in particular when a circular shape is processed by using X- and Y-axes, high machining accuracy is desired. In a conventional control system, it is difficult to improve the accuracy of the boundary between quadrants of an arc, due to delay of the servo, in particular when the servo is inversed.
On the other hand, as a method for realizing high speed and high accuracy, in response to repeating commands, a learning control (or a repeat control) may be used, which includes a time-based synchronization method wherein the learning is carried out by using time as a reference (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 3-175502) and an angle-based synchronization method wherein the learning is carried out by using an angle as a reference. For example, Japanese Unexamined Patent Publication (Kokai) No. 2004-280772 discloses learning control, wherein correction data corresponding to the position of the shape in repeat patterns is stored and the positional deviation is corrected corresponding to the position, by which the positional deviation can be reduced even when velocity fluctuation occurs.
Since the time-based synchronization method requires a memory delay for time from start to end of the processing, a high-capacity memory is necessary when processing time is relatively long. Further, when the same shape is processed at different processing speed, another memory delay is necessary. On the other hand, in the angle-based synchronization method, there is no disadvantage when the processing time is long or when the processing speed is varied, however, a reference angle (or a reference position), which monotonically increases (or increases in one direction) in synchronization with repeated commands, is necessary. Therefore, the reference position is varied like a reciprocating motion, the angle-based synchronization method cannot be used.
In the description regarding FIG. 3 of Japanese Unexamined Patent Publication (Kokai) No. 2004-280772, correction data δ(n) corresponding to reference position Θ(n) is calculated by interpolating calculation as shown in the following equation, wherein reference position Θ(n) is obtained at the time of sampling, and correction data, before and after reference position Θ(n), are δ(m) and δ(m+1) which correspond to grid positions θ(m) and θ(m+1), respectively.δ(n)=δ(m)+(Θ(n)−θ(m))·(δ(m+1)−δ(m))/θ(m+1)−θ(m))
When reference position Θ does not monotonically increase or decrease, a plurality of data δ may exist corresponding to each reference position, whereby the correction data cannot be accurately determined.